christie at the cape

6/14/2010

Well it's been a while since I blogged here - or anywhere - and since I'm no longer "at the cape" I'm retiring this blog. Maybe I'll pick it up again in the future. Anyway thanks for following and all your comments over the years, it's been fun. I'll add a contact email to my profile in case anybody wants to contact me with questions about any of my posts but won't check back here anymore.

3/05/2010

hello from windhoek. We have just packed our rocks (wrapped in plastic bags) in steel 20-L paint buckets and filled the gaps with expanding insulation foam. any bets on whether this will work? not sure if it will kick off properly inside the bucket. not sure if it will withstand the motion of the rocks in the truck south. if it works as well as it appears to - we have revolutionized the preservation of structural rock samples which so often I have lost to mechanical weathering along the way home...

1/03/2010

Geological terminology is always evolving. The meanings of old terms can change through time. Early suggested terms may turn out to be inaccurate and may be replaced, or may become more or less specific in their usage. Nevertheless, old habits die hard. Certainly the use of certain terms help us establish or test identity or affinities - scientific jargon at large plays important roles in social relationships.

4. Basement"Basement" is the ultimate relativistic term. It describes the rocks one is not interested in talking about. If you write a paper about soils, the recent sediments underneath may be the "basement". If you're writing about those sediments, the granite underneath those may be the basement. Basically it means whatever is older or under or around or near the rocks one is actually interested in. My friend Mike, who loves ice and lichen and moss and squirrels and everything else more than he loves rocks, gave me another variation on "basement" the other day when I accidentally picked him up on the Golden Gate Bridge: "underburden". Nice.

3. Subduction channelApologies to my good friend Åke, but this one has to die. The "subduction channel" refers to the area between the subducting lithosphere and the overriding plate in a subduction plate boundary, where sediments are subducted and variably deformed and metamorphosed and de-watered. It is generally seen as a tabular region - not linear - so it isn't the shape of a channel. The term also causes confusion due to some people suggesting models where material flows only downward through this "channel", while others invoke backflow within the "channel" to help uplift metamorphic rocks which record very deep conditions. Both camps use the term "channel" and neither one is describing a 1D feature. I'm just confused by this. Finally, in a channel, the flowing media flows in one direction relative to the walls of the channel, right? But in a "subduction channel" the flowing stuff (subducting sediments) moves in a direction and rate intermediate between the upper and lower walls. It's a shear zone. Not a channel.

2. Pseudotachylyte (or pseudotachylite)Aside from having two spellings (-yte is older and therefore preferred although it is counter-intuitive most of the time and looks weird with 2 "y"'s so close to each other), the term "pseudotachylyte" is an example of defining something by what it is not, instead of what it is. Pseudotachylyte is a glassy rock formed by either seismic or impact-related melting of any rock (but in practice is restricted to silicates). It does not therefore include a whole suite of other glassy rocks (igneous, melt cortices on meteorites, etc), so it is not a good descriptive term, but requires an interpretation of how the thing formed. Finally, tachylyte is an igneous glassy rock similar to obsidian. So pseudotachylyte is something that could be mistaken for tachylyte but is not. Oh yah, and there are other very fine grained, dark coloured fault rocks (e.g. ultramylonite, ultracataclasite) that can't be distinguished from pseudotachylyte without some serious microscopy. So it's not useful as a field term either. Fail.

1. Pan-AfricanAs far as I can devise from the literature, the term Pan-African refers to nearly any geological event (mostly magmatic but also metamorphic, deformational, etc) occurring during a period of approximately 250 million years (roughly 750-500Ma) anywhere in Africa or continents formerly associated with Africa. My dear colleagues who advocate the use of this term tell me the exact meaning can be deduced from the context of the specific location or events being discussed which makes this term actually less useful than not using any term at all. I can think of no good reason to use "Pan-African" at all unless one is trying to obscure the problem of massive dating errors or giant uncertainty about tectonic events. THIS ACTUALLY HAPPENS.

ok just to make me sound a little less cranky, here's a comment left by somebody called NJ on Kim's blog a while ago that makes me totally happy:

"You'd better wait. My desk is totally Franciscan right now and I have no idea where to start looking.

Or:

"He completely Franciscaned his first draft and his advisor wouldn't even read it."

12/19/2009

Every region has its particular strengths and weaknesses with regards to the type of geology which is easily accessible for student field trips. In the areas surrounding UCT, we have some seriously awesome geology but there are at least two things my students have to accept without seeing any really clear direct evidence in the field:

plates really move

plates really subduct.

Since I have a student from UCT with me in San Francisco this week for the AGU meeting (who gave a badass poster presentation by the way), it's a good opportunity to fill some of these gaps. We took an afternoon drive down to Hollister and San Juan Bautista to see some evidence for recent fault creep offsetting sidewalks and walls.

Hollister is positioned just north of the split where the Calaveras Fault branches from the main strand of the San Andreas Fault. The Calaveras Fault is creeping through Hollister, but rates vary along the fault in space and time from 3-18mm/yr (http://funnel.sfsu.edu/creep/SiteTable.htm). (Map from http://quake.usgs.gov/recenteqs/)

My student is clearly excited by this right-lateral bulge in a garage wall.

Tension gashes where the fault crosses the street at a high angle and disappears straight under the middle of a house.wonky sidewalkanother wonky sidewalkseriously wonky sidewalk, and the steep small hill on the left of the photo is a pressure ridgewonky sidewalkmore tension gashes in the streetcarly will be creeping to your right as you look at this photo.It's cool to see how different sidewalks and houses of different ages have accumulated different amounts of offset. We also couldn't help noticing new skirting and lots of concrete repairs which presumably addressed the larger offsets. Also - in some places the total offset was accommodated by narrow strands (usually ~ 1m wide) but in others, the deforming zone seemed to be much wider (10m). Seems like this depends on local soil conditions as well as the rigidity of the surface features. Sometimes it is wide under a sidewalk and sometimes all the strain seems to accumulate on one joint between sidewalk panels, as in the last photo.

Today we are off to Ring Mountain with Åke to see some evidence for #2.

9/30/2009

Finally back in CT for a while. Lots to post but more to catch up on. Here are some cool stromatolites in dolomites of the Naukluft Nappe complex. There's a thin layer of sand over the top of the carbonate bed. I wonder what this represents. Was it a wave that washed sand between the bioherms? Did it kill them? I didn't see the beds above. Isn't it incredible how the sedimentary record is a stack of discrete moments - not a continuous record. Just snapshots.

I love being a geologist because I can hike up a cliff on a dry hot windy day in southern Africa, watch a meerkat shading himself with his tail, scare a herd of Hartman's Mountain Zebra up the slope ahead of me, then sit on this 550-million year old warm shallow sea and imagine a tropical, tectonically active world owned completely by algae and possibly some ediacaran fauna - no shells, no teeth, no fish, no birds. Must have been a quiet and peaceful world.

9/14/2009

The traditional "First Fold" picture - see 2008 and 2007). I just can't get enough of this cool folded bed in the lower Prince Albert Formation.This class was fun - and every one of them was pretty keen in the field! Also did their chores without nagging! Truly a first on both counts. It was a nice one to go out on - my last trip to Laingsburg, at least as a lecturer at UCT.

It was a big year for transitions - as Dr. John Rogers, our sedimentologist with whom I have taught this part of the field course for the last 4 years, will be retiring at the end of this year.We were lucky to have the two new guys (our replacements) accompanying us for the trip, in a kind of hand-over. It was great to be in the field with them and see all the energy and interest and excitement they will bring to the department. They both saw a lot of research potential in the area too - I hope some of their plans will turn into future Honours projects for these students!

As you can see, they worked well together in the field. All the groups did. I haven't seen their final maps yet but I have a feeling they will be good.Every year I change it up a little bit - we usually do a "structure training day" and a "sedimentology training day" before they start mapping on their own. This year it went particularly well. I decided to focus directly on field methods instead of rehashing the structure topics we discussed in the classroom. We practiced sketching from afar and ground-truthing the sketch, and talked a lot about scale and planning where to go. Here's an example of a student with his field sketch of a faulted anticline thrusted over a faulted anticline. That peaky Prince Albert Formation sure does take up a lot of the strain in this part of the fold belt.

Here we are on the last day - the last day, for me, of formal teaching at UCT. Pretty sad about that but also looking forward to the next phase of my life.

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